Silicon tetraisocyanate and various organosilyl isocyanates have classically been prepared by heating a silicon tetrahalide or an organosilyl halide with silver isocyanate in an inert organic solvent such as benzene. See, for example, J. Am. Chem. Soc., 70, 1042 (1948), J. Am. Chem. Soc., 70, 1222 (1948) and J. Am. Chem. Soc., 72 196 (1950), and references cited therein.
J. Org. Chem., 28, 586 (1963) discloses, inter alia, the preparation of silicon tetraisocyanate, trimethylsilyl isocyanate and dimethylsilyl diisocyanate by reacting the appropriate silyl halide with isocyanic acid (prepared by cracking isocyanuric acid at about 600.degree. C.) in an inert solvent, using trimethylamine or pyridine as a proton acceptor.
U. K. Patent No. 643,941 discloses, inter alia, the preparation of methylsilyl isocyanates by heating the appropriate methylsilyl halide with an alkali metal cyanate in a pressure vessel at a temperature of about 300.degree.-335.degree. C.
J. Am. Chem. Soc., 72, 3045 (1950) describes the preparation of triphenylsilyl isocyanate by (A( fusing triphenylsilyl chloride with urea and (B) heating triphenylsilyl chloride with silver cyanate or sodium urethan in an inert organic solvent.
Chem. Ber., 93, 1111 (1960) discloses the preparation methylsilyl isocyanates by the reaction of the appropriate methylsilyl halide (A) with lead cyanate in an inert solvent, (B) with potassium cyanate in an inert solvent with concentrated sulfuric acid or acetic acid as catalyst and (C) with urea at a temperature of from 280.degree. C. to 340.degree. C. and at pressures of from 45 to 105 atmospheres.
West German DAS No. 1,205,099 discloses the preparation of organosilyl isocyanates of the formula EQU R.sub.n Si(NCO).sub.4-n
wherein R is (lower)alkyl, (lower)alkoxy or phenoxy and n is 2-3 by the reaction of an organosilyl halide of the formula EQU R.sub.n SiX.sub.4-n
in which X is halogen, with an alkali metal cyanate in liquid sulfur dioxide at a temperature of about -25.degree. C. to 70.degree. C. Above the boiling point of sulfur dioxide (-10.degree. C.) the reaction must be conducted in a pressure vessel.
U. K. Pat. No. 1,373,291 discloses the preparation of organic isocyanates (such as benzyl isocyanate and xylylene diisocyanate) by the reaction of the appropriate organic halide (in which the halogen must be aliphatically bound) with an alkali metal cyanate, an alkaline earth metal cyanate or an ammonium cyanate in the presence of a crown ether. The reaction may be conducted in the absence of solvent or in an inert polar or non-polar organic solvent, or a mixture thereof.
West German OLS No. 1,965,741 discloses, inter alia, the preparation of organosilyl isocyanates of the formula EQU R.sub.n Si(NOC).sub.4-n
wherein R is alkoxy, aryloxy, alkenyl, SiO-, or substituted or unsubstituted alkyl, aralkyl or aryl, and n is 0-3, by reacting an organosilyl halide of the formula EQU R.sub.n SiX.sub.4-n
wherein R and n are as above and X is halogen, with an alkali metal cyanate or an ammonium cyanate in the presence, as catalyst, of a polar organic solvent having a dielectric constant of at least 10, e.g. dimethylformamide, dimethylacetamide, N-methylpyrrolidone cyclohexanone or benzonitrile. The polar solvent may be used in admixture with an inert non-polar organic solvent.
It will be appreciated that, although numerous procedures for the preparation of silyl isocyanates have been described, most of these involve the use of expensive chemicals (such as silver cyanate), high cost or otherwise generally undesirable technology (such as high temperature, high pressure, use of liquid sulfur dioxide, or the like) or low yield.
It is known that certain two-phase reactions between active halides (generally organic halides) and nucleophiles are catalyzed by quaternary ammonium salts, quaternary phosphonium salts and by crown ethers; such reactions are referred to as examples of phase transfer catalysis. The field of phase transfer catalysis is fairly new and not fully understood. Thus, no general rules have been formulated which allow one to identify reactions which will proceed by phase transfer catalysis. Many factors such as substrate structure, nucleophile, solvent, phase transfer catalyst structure and reaction conditions all appear to play a part in unpredictable ways. Furthermore, the successful applications of phase transfer catalysis are largely in the field of organic chemistry. These appear to be no examples in which alkali metal cyanates have been successfully reated with silyl halides under phase transfer catalysis conditions to give silyl isocyanates. Indeed, early attempts by us to use quaternary ammonium type phase transfer catalysts to prepare silyl isocyanates failed. Thus, when trimethylchlorosilane and sodium or potassium cyanate were stirred in toluene in the presence of tetrabutylammonium iodide, no trimethylsilyl isocyanate could be detected in the reaction mixture. To our surprise, however, when a small amount of 18-crown-6 ether was added to the reaction mixture a very vigorous reaction took place leading to a virtually quantitative yield of trimethylsilyl isocyanate.
A review of phase transfer catalysis, including the use of crown ethers as phase transfer catalysts, is found in Angewandte Chemie, International Edition, 16, 493 (1977). A review of the principles and applications of crown ether chemistry is found in Synthesis, 1976, 168-184.